Electrodes for use in aqueous alkali metal chloride electrolytes

ABSTRACT

THE INVENTION RELATES TO A PLATINUM METAL COATED TITANIUM ELECTRODE FOR USE IN ELECTROLYSIS OF BRINE. BY ELECTROPLATING THE PLATINUM GROUP METAL FROM AN ELECTROLYTE CONTAINING A POLYBASIC, HYDROXY ORGANIC ACID THE RESULTING PLATED ELECTRODE IS &#34;ACTIVATED.&#34;

3,578,572 Patented May 11, 1971 ELECTRODES FOR USE IN AQUEOUS ALKALI METAL CHLORIDE ELECTROLYTES Denis Lee, Runcorn, England, assignor to Imperial Chemical Industries Limited, London, England No Drawing. Filed Oct. 21, 1968, Ser. No. 769,400 Claims priority, application Great Britain, Nov. 10, 1967, 51,219/ 67 Int. Cl. Clb 11/26; C23b 5/24, 5/48 US. Cl. 204-95 9 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a platinum metal coated titanium electrode for use in electrolysis of brine. By electroplating the platinum group metal from an electrolyte containing a polybasic, hydroxy organic acid the resulting plated electrode is activated.

The present invention relates to the manufacture of electrodes for use in aqueous alkali metal chloride electrolytes. More particularly it relates to the manufacture of coated titanium electrodes having an activated surface of a platinum metal with low chlorine-overvoltage characteristics.

In recent years it has been proposed to replace the conventional graphite anodes used in alkali metal chloride electrolytes, especially in cells electrolysing alkali metal chloride solutions for the manufacture of chlorine, hypochlorites and chlorates, by anodes comprising a core of titanium carrying a coating of a platinum metal, particularly platinum itself. In order to reduce the capital cost of these electrodes to an economically attractive level it is necessary to employ very thin layers of platinum, and these thin layers can most conveniently 'be produced by electrodeposition. Electrodeposition can easily be controlled to produce uniform thin coatings, the process operates at relatively low temperatures so that there is no danger of distorting the electrode structure by heating, and electrodeposited coatings have good resistance to wear in use, even as anodes in mercury-cathode cells Where contact with mercury can damage platinum coatings. Although for these reasons, the electrodeposition method is attractive, it has one serious disadvantage in that platinum deposits from conventional platinum-plating baths exhibit a high overvoltage for the liberation of chlorine when taken into service as anodes in alkali metal chloride electrolysis and the overvoltage quickly rises to even higher levels as electrolysis proceeds.

We have now found that this disadvantage can be overcome if an admixture of one of a group of organic acids is made to the electroplating bath from which at least an outermost layer of the platinum metal is electrodeposited.

According to the present invention we provide a method for the manufacture of an electrode comprising a titanium support and an electrodeposited coating of a platinum metal thereon, which is characterised in that at least an outermost layer of the platinum metal coating is electrodeposited from a plating bath which contains an admixture of an aliphatic polybasic hydroxy acid.

In the preceding paragraph by a titanium support We mean a support consisting of titanium alone or an alloy based on titanium and having anodic polarisation prop erties comparable with those of titanium. Examples of suitable alloys are titanium-zirconium alloys containing up to 14% of zirconium, alloys of titanium with up to 5% of a platinum metal such as platinum, rhodium or iridium and alloys of titanium with niobium. or tantalum containing up to 10% of the alloying constituent. By a platinum metal we mean one of the metals ruthenium, rhodium, palladium, osmium, iridium, platinum or an alloy of at least two of these metals.

In carrying out the method of the invention the amount of polybasic hydroxy acid employed may vary over a wide range. These very soluble acids prevent electrodeposition entirely if used at high concentration and in general they should not be used at more than half the concentration which prevents electrodeposition. This limit can readily be determined by simple trial. The most useful range is generally 10-100 grams of the acid per litre of the plating bath. The preferred polybasic hydroxy acid is citric acid and a very suitable concentration is approximately 20 g./l.

The whole of the platinum metal coating of the electrode may be deposited on the titanium support from a plating bath containing the polybasic hydroxy acid. However, within the scope of the invention the titanium support may first be given an undercoating of a platinum metal by electrodeposition from a conventional plating bath and only a thin surface layer of platinum metal need then be deposited over the undercoating from a plating bath containing an admixture of the polybasic hydroxy acid to produce an activated" surface in accordance with the broad concept of the invention.

We have found that the electrodes produced according to the invention are particularly useful as anodes in aqueous alkali metal chloride electrolytes since the chlorine overvoltage at the platinum surface is low at the start of electrolysis and does not increase appreciably with time. The electrodes may be employed, for example, as anodes in electrolytic cells wherein chlorine, hypochlorites or chlorates are produced by the electrolysis of alkali metal chloride solutions, as anodes in cells for the electro-dialysis of brackish water and as anodes for the cathodic protection of iron and steel structures immersed in sea water.

The invention is illustrated but not limited by the following comparative examples:

EXAMPLE 1 (Comparative example outside the scope of the invention) Platinum was electrodeposited onto a sheet of titanium which had been etched in 35% hydrochloric acid by making the etched titanium the cathode in a platinum-plating bath containing hydrochloric acid at 1 molar concentration and l g./l. of chloroplatinic acid, using a current density of 0.3 A./dm. for 10 minutes with the bath temperature at C. The titanium sheet cathode was kept in motion in the bath with the plane of the sheet tangential to a circular path and a linear velocity of cm./ minute.

EXAMPLE 2 Platinum was electrodeposited onto etched titanium exactly as in Example 1 except that the plating-bath also contained 20 g./l. citric acid.

Each of the coated titanium specimens 1 and 2 was tested for development of chlorine-overvoltage by making it the anode in sodium chloride brine saturated with chlorine and containing 250 g./l. NaCl at a pH of approximately 2 and a temperature of 65 C. The potential of the Overpotential at 4 kA.,/m.

After After Initial, hours, 70 hours,

mv. rnv. mv

Example (specimen):

What I claim is:

1. A method for the manufacture of an electrode comprising a titanium support and an electrodeposited coating of a platinum metal thereon, which is characterised in that at least an outermost layer of the platinum metal coating is electrodeposited from a plating bath which contains an admixture of an aliphatic polybasic hydroxy acid.

2. A-method according to claim 1, wherein before the outermost layer of the platinum metal is electrodeposited the titanium support is provided with a coating of a platinum metal by electrodeposition from a plating bath which does not contain the said admixture of an aliphatic polybasic hydroxy acid.

3. A method according to claim 1, wherein the said 4 admixture is in the range of 10-100 grams of the said acid per litre of the bath.

4. A method according to claim 1, wherein the said acid is citric acid.

5. A method according to claim 4, wherein the amount of citric acid employed is approximately 20 grams per litre of the bath.

6. A method according to any of the preceding claims wherein the platinum metal of the coating is platinum.

7. An electrode comprising a titanium support and a coating of a platinum metal thereon prepared by the method of claim 1.

' 8. A process for the electrolysis of an aqueous solution of alkali metal chloride, characterised in that there is used as anode in said electrolysis an electrode according to claim 7.

9. A process according to claim 8, wherein the alkali metal chloride is sodium chloride.

References Cited UNITED STATES PATENTS 2,057,476 10/ 1936 Cohn 20447 2,335,321 11/1943 Wise et a1. 204-47 3,055,811 9/1962 Ruff 20447X FOREIGN PATENTS 177,875 4/ 1922 Great Britain 204-47 F. C. EDMUNDSON, Primary Examiner US. Cl. X.R. 204-47, 290 

